The present invention relates to an apparatus and method for trapping ions, at atmospheric pressure, within a defined 3-dimensional space, based on the ion focussing principles of high field asymmetric waveform ion mobility spectrometry.
High sensitivity and amenability to miniaturization for field-portable applications have helped to make ion mobility spectrometry an important technique for the detection of many compounds, including narcotics, explosives, and chemical warfare agents (see, for example, G. Eiceman and Z. Karpas, Ion Mobility Spectrometry (CRC. Boca Raton, Fla. 1994); and Plasma Chromatography, edited by T. W. Carr (Plenum, N.Y., 1984)). In ion mobility spectrometry, gas-phase ion mobilities are determined using a drift tube with a constant electric field. Ions are gated into the drift tube and are subsequently separated based upon differences in their drift velocity. The ion drift velocity is proportional to the electric field strength at low electric fields (e.g., 200 V/cm) and the mobility, K, which is determined from experimentation, is independent of the applied field. At high electric fields (e.g. 5000 or 10000 V/cm), the ion drift velocity may no longer be directly proportional to the applied field, and K becomes dependent upon the applied electric field (see G. Eiceman and Z. Karpas, Ion Mobility Spectrometry (CRC. Boca Raton, Fla. 1994); and E. A. Mason and E. W. McDaniel, Transport Properties of Ions in Gases (Wiley, N.Y., 1988)). At high electric fields, K is better represented by Kh, a non-constant high field mobility term. The dependence of Kh on the applied electric field has been the basis for the development of high field asymmetric waveform ion mobility spectrometry (FAIMS), a term used by the inventors throughout this disclosure, and also referred to as transverse field compensation ion mobility spectrometry, or field ion spectrometry (see I. Buryakov, E. Krylov, E. Nazarov, and U. Rasulev, Int. J. Mass Spectrom. Ion Proc. 128. 143 (1993); D. Riegner, C. Harden, B. Carnahan, and S. Day, Proceedings of the 45th ASMS Conference on Mass Spectrometry and Allied Topics, Palm Springs, Calif., Jun. 1-5, 1997, p. 473; B. Carnahan, S. Day, V. Kouznetsov, M. Matyjaszczyk, and A. Tarassov, Proceedings of the 41st ISA Analysis Division Symposium, Framingham, Mass., Apr. 21-24, 1996, p. 85; and B. Carnahan and A. Tarassov, U.S. Pat. No. 5,420,424). Ions are separated in FAIMS on the basis of the difference in the mobility of an ion at high field Kh relative to its mobility at low field K. That is, the ions are separated because of the compound dependent behaviour of Kh as a function of the electric field. This offers a new tool for atmospheric pressure gas-phase ion studies since it is the change in ion mobility and not the absolute ion mobility that is being monitored.
One application of this tool as realized by the present inventors is in the area of ion trapping. To the inventors"" knowledge, there are no previously known devices or methods that produce any sort of a 3-dimensional ion trap at atmospheric pressure (about 760 torr). While other 3-dimensional ion trapping mechanisms do exist, these known ion traps are typically designed to operate below 1 torr, in near-vacuum conditions. The efficiency of these ion traps degrades extremely rapidly as the pressure increases beyond 10 torr, and there is no experimental or theoretical basis to suggest that any trapping occurs, using these known methods, at 760 torr.
In one aspect, the present invention provides an apparatus for selectively transmitting ions and trapping said ions within a defined 3-dimensional space, comprising:
a) at least one ionization source for producing ions;
b) a high field asymmetric waveform ion mobility spectrometer, comprising an analyzer region defined by a space between at least first and second spaced apart electrodes for connection, in use, to an electrical controller capable of supplying an asymmetric waveform voltage and a direct-current compensation voltage for selectively transmitting a selected ion type in said analyzer region between said electrodes at a given combination of asymmetric waveform voltage and compensation voltage, said analyzer region having a gas inlet and a gas outlet for providing, in use, a flow of gas through said analyzer region, said analyzer region further having an ion inlet for introducing a flow of ions produced by said ionization source into said analyzer region; and
c) a curved surface terminus provided on at least one of said electrodes, said terminus being a part of said one of said electrodes which part is closest to said gas outlet, said defined 3-dimensional space being located near said terminus, whereby, in use, said asymmetric waveform voltage, compensation voltage and gas flow are adjustable, so as to trap said transmitted ions within said 3-dimensional space.
Said first and second electrodes may comprise curved electrode bodies to provide a non-constant electric field therebetween, whereby, in use, said ions are selectively focussed in a focussing region created between said curved electrode bodies in said analyzer region.
In another embodiment, said first and second electrodes comprise outer and inner generally cylindrical coaxially aligned electrode bodies defining a generally annular space therebetween, said annular space forming said analyzer region, and said terminus being provided at an end of said inner cylindrical electrode body.
In another aspect, the present invention provides a method for selectively transmitting and trapping ions within a defined 3-dimensional space, said method comprising the steps of:
a) providing at least one ionization source for producing ions;
b) providing an analyzer region defined by a space between at least first and second spaced apart electrodes, said analyzer region being in communication with a gas inlet, a gas outlet and an ion inlet, said ions produced by said ionization source being introduced into said analyzer region at said ion inlet;
c) providing an asymmetric waveform voltage and a direct-current compensation voltage, to at least one of said electrodes;
d) adjusting said asymmetric waveform voltage and said compensation voltage to selectively transmit a type of ion within said analyzer region;
e) providing a curved surface terminus on at least one of said electrodes, said defined 3-dimensional space being located near said terminus; and
f) providing a gas flow within said analyzer region flowing from said gas inlet to said gas outlet and adjusting said gas flow to trap said transmitted ions within and near said defined 3-dimensional space, said gas outlet being located near said terminus.
Advantageously, said analyzer region is operable substantially at atmospheric pressure and substantially at room temperature.
The method may further comprise the step of providing an ion outlet and supplying an extraction voltage at said ion outlet for extracting said trapped ions, said ion outlet being substantially aligned with said terminus and said defined 3-dimensional space.
In yet another aspect, the present invention provides an apparatus for selectively focussing ions and trapping said ions within a defined 3-dimensional space, comprising:
a ) at least one ionization source for producing ions;
b) a segmented high field asymmetric waveform ion mobility spectrometer, comprising an analyzer region defined by spaces between a plurality of corresponding pairs of first and second spaced apart electrodes, for connection, in use, to an electrical controller capable of supplying an asymmetric waveform voltage, a direct current compensation voltage and a direct current segment offset voltage, each of said plurality of corresponding pairs of first and second spaced apart electrodes forming a segment and said segments being aligned in a row immediately adjacent to and electrically isolated from each other, said analyzer region having an ion inlet for introducing a flow of ions produced by said ionization source into said analyzer region. In yet another aspect, the present invention provides a method of selectively focussing ions and trapping said ions within a defined 3-dimensional space, comprising the steps of:
a ) providing at least one ionization source for producing ions;
b) providing an analyzer region defined by spaces between a plurality of corresponding pairs of first and second spaced apart electrodes and providing a non-constant electric field between said first and second electrodes, each of said plurality of corresponding pairs of first and second spaced apart electrodes forming a segment and said segments being aligned in a row immediately adjacent to and electrically isolated from each other, said analyzer region being in communication with an ion inlet, and introducing said ions produced by said ionization source into said analyzer region at said ion inlet;
c) supplying an asymmetric waveform voltage to one of said first and second spaced apart electrodes in each of said segments;
d) supplying a direct current compensation voltage to said one of said first and second spaced apart electrodes in each of said segments, said direct current compensation voltages supplied to each of said segments being independently adjustable;
e) supplying a direct current segment offset voltage to another of said first and second spaced apart electrodes in each of said segments, said direct current segment offset voltages supplied to each of said segments being independently adjustable; and
f) adjusting said direct current compensation voltages and said direct current segment offset voltages substantially equally, thereby providing a constant direct current potential across each corresponding pair of first and second electrodes in each of said segments, so as to focus desired ions between each corresponding pair of first and second electrodes in each of said segments at a given combination of said asymmetric voltage, direct current compensation voltage, and direct current segment offset voltage.